I measured the stability of GPS disciplined 8040c using the three cornered hat method (THC). The other two clocks were rubidium standards that were not locked to GPS. The measurements were done using 5115A phase noise test set.
The calculated ADEV result of the 8040c+GPS is an almost constant horizontal line from tau=1s to tau=1000s located between 9E-13 and 1E-12 ADEV. After tau=1000s it starts to trend downwards (increasing stability), almost tracking the stability of the GPS receiver. The 8040c's tau=1s calculated stability is about the same regardless if it's disciplined to GPS or not (this is normal of course as a rubidium standard should be more stable in short term than GPS). The results are radically better that what the 8040c specifications state for its short term stability. I have not measured the GPS 1PPS output, the numbers I quoted are the specs. Unless specifically said otherwise, I always assume that the specs are stated with a coverage factor of two which is the GUM recommendation and my anecdotal experience of manufacturers' habits. Basically what you two are saying is that the 8040c's 1E-12 GPS disciplined spec is worthless and it shouldn't be used under any circumstances when determining the uncertainty of the whole system? My personal take is this: The 8040c determines itself whether or not the disciplining 1PPS signal it is receiving is of good quality and it does not lock to a bad quality signal. The specifics of the locking mechanism are described in the SA.22C-LN designer's manual. Therefore, if the 1PPS signal is qualified by the 8040c and we make the assumption that the device is working as intended, we can justifiably trust the spec and classify it as a Type B uncertainty. We can make the assumption that the 8040c's internal oscillator is working as intended and it is good for qualifying the 1PPS input because the three cornered hat *without* 1PPS lock yields good results (TCH with and without 1pps lock are basically identical up to tau=1000s). My master's thesis goal was to determine the frequency accuracy and stability of the GPS+8040c and the conundrum being that it seemed to be best clock at the lab. Therefore I did not have the possibility to measure it against a better - for example cesium - standard. I use past tense here because at the moment I'm just writing the thesis and all the measurements are done. I do not have the possibility of doing more measurements as I don't have access to the equipment anymore. I apologize but I would like to preserve some anonymity so I'm hesitant to declare my school. My major is measurement technology but this frequency standard field is a little bit outside my expertise (which is good for learning I guess). I realize now that I should have come to this place sooner as you seem to know your stuff pretty well and have great ideas that I hadn't thought before :) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
